In my experience Gliclazide alone won't keep bgs down. I'm on a very low carb diet which in addition to Gliclazide helps. If I forget to take the med then my bgs shoot up pretty quickly.
As has been said there could be a myriad of reasons why your bgs have risen. T2 diabetes behaves in a non linear way so it might be wise not to react to a few negative results. However if it continues it might be the time to tweak your diet and see what happens.
Do you do any exercise such as walking, swimming, gym etc?